Question

Absolute refractive index of a substance is
(A). Speed of light in vacuum/ Speed of light in medium
(B). Speed if light in water/ Speed of light in medium
(C). Speed of light in medium/ Speed of light in air
(D). Speed of light in medium/ Speed of light in water

Answer 1

- Hint: Absolute refractive index relates the refractive index of a material to that of the refractive index of vacuum, hence absolute.

Complete step-by-step solution -

The bending of light-ray from its original path when it passes from one medium to another is called refraction. The amount and direction of bend is quantified by the refractive index of the materials. Refractive index is a relative property of the two media. It is given by,
$_{1}{{n}_{2}}=\dfrac{{{v}_{1}}}{{{v}_{2}}}=\dfrac{\text{Speed of light in the first medium}}{\text{Speed of light in second medium}}$
Again,
$_{1}{{n}_{2}}=\dfrac{{{v}_{1}}}{{{v}_{2}}}=\dfrac{{{n}_{2}}}{{{n}_{1}}}$
where, $n_1$ and $n_2$ are the absolute refractive index of the first and second medium respectively. If the first medium is vacuum, for which $n_{vac}=1$, the absolute refractive index of the second medium $n_{med}$ is given by,
$_{vac} n_{med} = \dfrac{n_{med}}{n_{vac}}=n_{med}=\dfrac{c}{v}$
where, c and v is the velocity of light in vacuum and the given medium respectively.
Hence, the absolute refractive index of a substance is the ratio of Speed of light in vacuum to speed of light in medium.

Therefore, the correct option is A.

Note: The refractive index is also defined by the ratio of sine of angle of incidence and sine of angle of refraction. In the process of refraction, the velocity of light changes, along with its wavelength and the intensity (due to finite reflection). But its frequency remains unchanged. Absolute refractive index of any medium other than vacuum is greater than 1, owing to the fact that the velocity of light in any medium is less than that in vacuum.